The ability to convert transient stimuli from the extracellular environment into long-term changes in neuronal function is central to an animal's capacity to adapt and learn from its environment. This is mediated through sensory organs which transduce physical and chemical stimuli into precise patterns of neuronal activity that elicit specific changes in the structure and function of the nervous system. Insight into the mechanisms that underlie these activity-dependent changes has been facilitated by the discoveries of many laboratories over the last several decades demonstrating that neurotransmitters released at neuronal synapses drive proteasome dependent protein degradation (J Biol Chem 284, 26655 (2009); Nat Neurosci 6, 231 (2003)). Consistent with a role for neural activity in regulating protein degradation, the proteasome localize to sites of synaptic activity (Nature 441, 1144 (2006)). This regulation is central to the ability of a neuron to appropriately respond to stimuli, as inhibition of protein degradation impairs a host of neuronal functions, ranging from plasticity at the Aplysia sensorimotor synapse to cell migration, neurotransmission, and physiology in the mammalian nervous system (Neuron 32, 1013-1026, 2001; Neuron 52, 239-245, 2006; Cell 89, 115-126, 1997; J Neurosci 26, 11333-11341, 2006) including the maintenance of long-term potentiation, a critical cellular mechanism underlying learning and memory (Neuron 52, 239 (2006); Nat Neurosci 9, 478 (2006)). Moreover, mutations in components of protein degradation machinery cause profound defects in human cognitive function (Biochim Biophys Acta 1843, 13 (2014); Nat Rev Genet 8, 711 (2007)).
However, roles for proteasome function in the nervous system are more complex than they may appear. Proteasome function is required for certain aspects of nervous system function over long timescales (hours to days), such as synaptic remodeling and cell migration (Nat Neurosci 6, 231-242, 2003; Science 302, 1775-1779, 2003). Contrastingly, proteasome function is also required for activity-dependent neuronal processes over very short timescales (seconds to minutes), such as regulating the speed and intensity of neuronal transmission or the maintenance of long-term potentiation (Nature 441, 1144-1148, 2006; Neuroscience 169, 1520-1526, 2010; J Biol Chem 284, 26655-26665, 2009; Learn Mem 15, 335-347, 2008; J Neurosci 26, 4949-4955, 2006; J Neurosci 30, 3157-3166, 2010).
Proteasomes are heterogeneous multisubunit catalytic complexes that consist of a core 20S stacked ring of α/β subunits with a α7β7β7α7 architecture, and can be associated with 19S or 11S regulatory cap-particles to form a 26S proteasome (Ann. Rev Biochem 65, 801-847, 1996). While the natural behavior of 26S capped proteasomes is to mediate ATP-dependent degradation of ubiquitinated proteins, 20S uncapped proteasomes do not require ubiquitin or ATP for their catalytic function (Biomolecules 4, 862-884, 2014; EMBO J 17, 7151-7160, 1998; Proc Natl Acad Sci USA 95, Proc Natl Acad Sci USA 95, 2727-2730 2727-2730, 1998) Recent studies have shown that 20S proteasomes may have key biological functions separate from the canonical 26S ubiquitin-proteasome degradation pathway, particularly in clearing unstructured proteins and in degrading proteins during cellular stress (Ben-Nissan and Sharon, 2014). Despite extensive studies on proteasome function in neuronal signaling, the role of the 20S proteasome in the nervous system has remained unknown.
Critically, the functional studies addressing the role for proteasomes in the nervous system have either failed to discriminate between 20S and 26S proteasomes through the use of pan-proteasome inhibitors such as MG-132 or lactacystin, or have focused on the 26S proteasome through altering the ubiquitination pathway. Despite these and other efforts to understand the role of proteasomes in the nervous system, distinct proteasomes that potentially function independent of their proteostatic role to mediate rapid neuronal signaling have not been discovered. Therefore, we considered that taking an unbiased approach to evaluating proteasomes in the nervous system, without bias for 20S or 26S proteasomes, would provide a means to identify unique proteasomes that could possibly have acute signaling functions.
There exists an unmet need for better understanding protein degradation in neurons and its link to cognitive function and neuronal signaling in health and disease.